Noble-metal alloys adapted for ceramic bonding (the application of a porcelain jacket or covering) are well known in dentistry, and are used in the manufacture of crowns, bridges, and other prosthetic appliances and restorations used to replace damaged or missing teeth. These so-called "ceramic alloys" typically include about 80-90% gold and relatively high amounts of platinum (5-15%) and palladium (1-10%). Alloys of this type, and application of the alloys in dentistry, are discussed in detail in U.S. Pat. No. 3,413,723--Wagner and Pralow issued Dec. 3, l968, and for brevity, the disclosure of this patent is incorporated herein by reference.
A problem with known alloys is that they tend to present a somewhat silvery or white-gold appearance, whereas an ideal ceramic alloy should have a true yellow-gold color which remains stable after casting and through repeated firing cycles needed during the application of opaque and porcelain materials. The yellow-gold color is especially desired to match the color of adjacent gold crowns, and because most patients consider this color to be more esthetically and cosmetically pleasing.
I have found that the inclusion of about 1.5% to 3.0% rhodium in the alloy provides a surprisingly true and stable yellow-gold color. Small amounts of rhodium (less than 0.5%) have been previously used for alloy grain-refining purposes, but these quantities of the element are insufficient to produce a stable gold color. Another important factor is to limit copper content of the alloy to under 1.0%. Excessive copper has been found to produce severe oxidation when the alloy is cast, and to adversely affect bonding and color of the porcelain.